Most data encryption systems operate on the binary data produced by the data source. The data may be encoded for error detection andor correction before or after the encryption. The encryption is often performed by scrambling the data sequence with a “key” data sequence in order to conceal the original data sequence. Attempts may be made to alter the statistics of the encoded signal to make the signal appear more like random noise. After the encryption, the encrypted signal may be transmitted over a data path, or stored in a storage medium for later use. The transmission over a data path or storage may be viewed as a “channel” by which the encrypted signal is made available at a different time or venue. In any case, conventional digital signal processing techniques appropriate for the channel in question are used to aid in the transmission over the channel. Such techniques may include framing/packetizing, interleaving, modulating, filtering, and amplification. At the receiving end of the channel, the reverse operations are performed, in order to retrieve the original binary sequence.
An unauthorized or unintended entity which gains access to the encrypted signal by interception from the channel can apply various techniques to the encrypted signal, in an attempt to extract the original binary sequence.
U.S. Pat. No. 5,101,432, issued Mar. 31, 1992 in the name of Webb, describes a digital signal encryption technique in which the encryptor is a finite-impulse-response (FIR) network in which the impulse response is varied in time as the signal is encrypted, so that the impulse response “rolls” between different values during encryption. As described therein, the FIR network encrypts the signal by transforming with a substantially continuous nonlinear complex function of frequency, or in other words it disperses the original signal in time. This dispersion results in irregular “random-appearing” variations in amplitude of the signal. The random-appearing variations are not random, however, but arise from the time dispersion. The magnitude and possibly the phase spectra of the transformed signal are then complicated non-linear functions of frequency. Time dispersion as suggested by Webb implies distortion in the frequency domain. The time dispersion which Webb imposes on the signal during encryption is “inverted” by a decrypting filter.
Improved signal encryption/decryption techniques are desired.